Method and apparatus for dielectric heating



May 23, 1950 GARD 2,508,382

METHOD AND APPARATUS FOR DIELECTRIC HEATING Filed Oct. 10, 1946 2 Sheets-Sheet 1 INVENTOR 680/ 95 Gard May 23, 1950 G. E. GARD METHOD AND APPARATUS FOR DIELECTRIC HEATING 2 Sheets-Sheet 2 Filed Oct. 10, 1946 e5 Gard the plates; .aand (4) cutting off the his Patented May 23, 1950 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR DIELECTRIC HEATING George E. Gard, Lancaster, Pa, assignor to Annstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Application October 10, 1946, Serial No. 702,559

16 Claims. 1

This invention relates broadly to h art of l c ric stin i, thenceins of a ma of dielectric. mater l by subjectin to a ighifr q errcy l ernatin electric volta e or fi between a pair oispaoecl elect-rose plates onion- ;posite sides of the mass. More particularly. the invention .relates to aidielectric heating, having provisions for (1) heating the 'el c tone lates in order toavoidicooline of the e the dielectric mass adjacent theretoand permit the mass'to behest-ted to substanti same temperature throughout; (2) dete hing thextemperature of the interior of the mass as it heated; (r3) controlling the high-frequency power supplied to the electrode, plates so that the, dielectric mass is heatedv at the same rate as ouency power es-soon :as themass has reached a predetermined ternperature.

In the.-processing of certain material, i. e., cork granules having a heat. activatahle binder rea plied thereto, confined. within a mold betwe n electrode p a es. by dielectric eating, it has been found that themass of. matelel is, not located to the same emperature t rou hout becaus f heat-l sses iromfl he 0 portion by ond ti t -tzhe wal s ofthe. .old. and th lectrode .iilotes. As a result, the out r portio of h a e not as will area as th in ri r, The density of their .ed pr duct vari s because M43115. temperatur di eren ial and. ch v ialower the quality oi he Jim; p duct, e. a, sh ets tor the manu c ure: o gask ts or the .11 e.

proces n se e mat a s whe b differnces-between the temp ratur f he interior 'Qflihfi mess nd the out r por i n th reof are minimized. withthe result that a more uniform ure is obtained and the finished product exhibits substantially uniform density... In a preferred embodiment and practice of the invention, I lheat the electrode .platesirom an external source to prevent them irom absorbing heat from the outer portions of the massof dielectric and thereby chilling the latter. The electrode plates are, convenientl provided with external heaters of any suitable type, preferably electric-resistanceqor .radiantIheaters. The electrode plates a acer in. t e mal capacity n conse- -quent1y ta definite time is required to .heat them to asgiven temperature. The heating of .a .dielectric mass by the dielectric efiec't, however, is. almost. instantaneous because heat is generated uniformly throughout the mass and no I have inventeden appara us and m thod for time is required for heat transfer or flow, In order to assure that the heating of the mass shall proceed as rapidly as possible so as to attain a short curing cycle and yet not outrun the heating of the plates (which would give rise to the objectionable non-uniform cure mentioned above), I utilize the temperature of the plates as a check in controlling the application of highf-requency power to effect dielectric heating of the In other. words, I heat the plates as rapidly as possible and restrain the heating of themess so that its temperature in rising keeps with that .of the plates.

My invention thus differs from previous proposal to control the heating of the electrode plates in accordance with the temperature attained by the mass of dielectric material. This procedure is wholly impractical for certain applications s-ince the thermal lag of the electrode vplates may be longer than the complete curing cycle for some materials. Due to the relatively large thermal capacity of the electrode plates, when heating them either by electric heaters or any other means, they require relatively long time interval to attain a given temperature. On the other hand, as stated above,jheating by dielectric effect isalmost instantaneous. It is thus not possible to maintain uniform temperature throughout the-flielectric mass by any conceiw able means of varying the power input into the electrode plates in accordance with the temperature of the mass,

Specifically, my invention includes means for controlling the average power output of the;

source of high-frequency voltage used to effect dielectric heating accordance with the temperature of the electrode plates. That is to say, the rate at which the temperature of the electrorle plates rises is utilized as a limit on the rateat which the temperature of the mess of dielectric material is raised so that uniform temperature conditions throughout the latter may be maintained. This arrangement allows the temperature rise in the dielectric mass to proceed at the same rate as the temperature rise in the electrode plates, thereby maintaining a uniform temperature rise throughout the mass of dielectric material.

In order to control the rate of heating of the d fllectric mass, '1 provide electrical means for measuring the temperature at -the interior thereof and control the power input to the mass accordingly' This may be clone manually elthough I ,prefer automatic temperature-responsive means, I also employ temperaturerresponsive means to terminate the application of the alternating electric field when the temperature of the dielectric mass has reached the desired final value.

Dielectric heating has been conducted heretofore on a time-controlled basis, on the assumption that the material being heated is uniform with respect to moisture content, percentage of binder, and other factors influencing the rate of heating. Such a procedure may require exact formulation of mixes and storage of material prior to heating in air-conditioned space, otherwise the final temperature attained in a fixed time interval may vary, for example, because of non-uniform moisture content of the material. According to the method of my invention, I eliminate all these variables by merely continuin to heat the mass by the dielectric effect, regardless of the time, until the desired final temperature is reached, whereupon I discontinue the supply of high-frequency power to the mass and permit it to cool. In this connection, it is to be noted that the rate of temperature rise of the mass, in some materials, is quite high toward the end of the heating stage. It thus becomes ery important to avoid overshooting of the temperature and the resultant injurious effect such as overcuring or charring.

One of the electrode plates used in dielectric heating is usually grounded while the other is at a potential above ground which may be several thousand volts, depending on the character of the material being processed and the alternating electric stress which it is desired to establish therein. This presents a serious problem in making connections to the electric heater for the high-voltage electrode plate because of the difficulty of isolating the high-frequency voltage from the circuit supplying current to the heater for the electrode plate. I overcome this difficulty by employing a tuning coil formed of tubular conductor, connected in parallel with the electrode plates and the mass of material therebetween,

the electrical constants of the circuit being adjusted to provide parallel resonance when the circuit is connected to a current source of appropriate high frequency such as megacycles. Because of the high frequency employed, the current in the tuning coil flows exclusively in the outer layers thereof due to skin effect, thus leaving the heater-supply circuit on the interior entirely isolated. I also provide electrical temperature-indicating and control means for the highvoltage plate and the mass of material being heated and bring the leads therefrom out through the interior of the tubular conductor forming the tuning coil.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawings illustrating a preferred embodiment of the apparatus. In the drawings,

Figures 1A and 13 together constitute a diagrammatic View showing a mold for confining the mass of material to be heated in vertical section,

the tuning coil and control system for the source of high-frequency alternating voltage;

Figure 2 is an enlarged central section through a tube adapted to be inserted into the mass of material, having temperature-responsive means therein;

Figure 3 is a portion of Figure 1A to enlarged scale; and

Figure 4 is a partial diagrammatic view similar to a portion of Figure 1A, showing a modification.

Referring in detail to the drawings, a mold It! in the form of a box open at the bottom and top is composed of wood side walls ll having upper and lower binding frames 12 of angle iron. The wood walls of the mold are preferably impregnated with ceresin wax to give them a dielectric loss factor greater than that of the material to be processed. A method of dielectrically heating material utilizing a mold having a higher loss factor than the material to be processed is disclosed and claimed in the copending application of George E. Gard, Serial l lo. 676,217, June 1946, and entitled Method of compensating for or preventing heat losses from material during dielectric heating thereof. This affords compensation for heat losses to the walls which would otherwise occur. Electrode plates l3 and I4 form removable top and bottom walls for the mold, serving to confine therebetween a mass of dielectric material l5, e. g., cork granules having a heat activatable binder thereon. Upper and lower peripheral conductors 13a and I40, extend around the mold and are electrically connected to the plates l3 and 14, respectively, by jumper strips I31) and Mb. These conductors serve to create a more intensive field within the mold walls than is created in the mass and serve to compensate for heat losses from the mass to the mold walls. This is more fully disclosed and claimed in the copending application of George W. Scott, Jr., Serial No. 678,214, filed June 21, 1946.

Each of the plates I? and I4 is maintained in position by a pair of pins 66 extending through alined holes in opposite side walls of the mold. The plates i3 and I4 have covers I! and 18 engaging the outer surfaces thereof. The pins [6 bear on the covers I1 and 18. The construction of the mold as thus far described, except for the covers ll and i8, is also disclosed and claimed in a copending application of George W. Scott, Jr., Serial No. 678,215 filed June 21, 1946.

High-frequency alternating voltage is applied across the plates i3 and 14 from any convenient source, to establish the alternating electric field in the mass it) necessary to heat it by the dielectric effect. A known type of thermionic oscillator indicated at 19 serves as a convenient source of high-frequency current. The oscillator output is applied to a circuit tuned to parallel resonance including the mass l5 which forms a condenser with the plates 13 and hi, and an inductance composed of a conductor of suitable character, such as a tuning coil 20. One side of the oscillator output circuit is connected through the con tacts of a shut-01f contactor 2| and a supply line 22 to an adjustable contact 23 engaging the coil 28. The other side of the oscillator output circuit is grounded at 24. One end of the coil 20 and the plate [4 are similarly grounded at 25.

As illustrated, the tuning coil 26 is a helix formed from a tubular conductor, e. g., copper tubing. In a specific embodiment, copper tubing having a wall thickness 01' .035 was used. The upper end of the coil has a flexible lateral extension 26 connected to the cover ll. The tubular conductor is fianged at its end and is secured to a boss 2? on the cover I! by a collar 28. The cover l'l' being of metal and in contact with the plate [3 under the pressure exerted by the confined mass 15, provides a good electrical connection from the extension 26 to the plate. A grounded shield S of wire mesh encloses the mold I0 and coil 20.

Electric heaters 29 of the radiant type are mounted on the outer faces of the plates I3 and (Her thepur'pose of hea'ting the latter to 'prevent loss of heat from the top and bottom portions of the mass l as its-temperature increases 'on-heating by the'dielectric effect. The heaters maybe of any desired construction. As illustrated, they comprise simply a coil of resistance wire wound on a support which may conveniently be a'metal strip sheathed with suitable insulation. Any desired number of such heaters may be mounted on each electrode plate. Current is supplied to the heaters on the plate It through leads 30' extending outwardly through a hole 3! in-the cover i8 and a shielding tube am depending therefrom. The leads 39 are connected through a switch 32 to a source of current '33 of ordinary voltage and frequency, 1. e., 220 volts, 60 cycles. Heating current is similarly supplied to the heaters-on plate I 3 through leads 34. These leads, in theform of insulated conductors, extend into andthrough the tubular conductor forming the coil and enter the cover I! through the boss '21. Since the high-frequency current in the coil 20, the cover 57 and plate 13 is confined to the exterior of these parts, substantially infinite impedance exists between the parallel resonance circuit and the conductors 34 supplying the heaters 29 of the plate [3, the potential of which isseveral thousand volts above'that of the ground "to which the plate [4 is connected, when the oscillator is operated.

I provide means-responsive to the tem erature "to which the plate it is heated by the heaters 29, in order to control the power output or" theoscillator (which is the same asthe power input to the mass l5) so that the latter will beheated up at substantially the same-rate as the plates. I provide the plate 13 with a thermocouple 35 as shown in Figure 3. Leads 35 from the thermocouple are brought-out through the tubular conductor forming the coil and are connected to an indicating device 3i such as a millivo-lt meter. I also provide the plate 55 with a resistor as a further means for obtaining the temperature of the plate. A lead St extends the efrom through the tubular conductor forming the tun- 'ingcoili The resistance o'fthe resistor it-increases directly with the temperature and thus be used to actuate an indicator or controller in 'ac- 'cordan'ce with the temperature of th'eplate i3. Theresistor is connected to a controller (referred to hereinafter) by the lead 3 5'. The hot junction 'o'f'th'e thermocouple "and the resistor are both connected tothe plate I S.

The plate also has a thermocouple for temperature indication connected by leads it to a millivolt meter i i.

In order to comparethe 'temperati'u'e of the mass 1 5 as it rises to the desired maximum value with-the temperatures of the plates 53 and I4, I provide a thermo-re'sponsive element in the form of a fine tube it closed at one end, adapted to be inserted through one of a plurality of holes *43 in one of the side walls-of the mold it and to extend inwardly of the mass it to a point adjacent the center thereof. A-hermocouple and control resistors and are disposed in the tube 12 adjacent to the closed end as shown in Figure'2; Leads from the tl' ri ocouple and leads 4? and it" fro'inthe resistors through a flexible and extensible conducting case 48 branching from the tubular conductor forming the coil 29. The copper lead from the couple 44 serves as the common return for resistors 55 and 45 The case ts may conveniently be of bellowsilke construction. The tube 12 is preferably ins'erted so that it lies in the median'transvers plane through the mass [5 and parallel to the plates :3 and Id. Under these conditions, if the case 48 is connected to the mid-point of the coil 26, the potential at the end of the tube 42 will be the same as that on the point of the coil 20 to which the case 48 is connected, since the voltage gradient across the mass !5 isthe same as that across the coil 2-9, and there will be no flow of current which would heat the tube and thermo-responsive means and cause the-indication of the thermo-responsivemeans to be high. The tube .2, of course, may be inserted in the mass at any desired transverse plane if the point oi" connection of the case 48 to the coilZil is made to correspond so as to balance the voltage on the two sides of the electrical bridge composed of the capacities between the tube 52 and the plates l3 and ii on the one side,- and the portion of the coil 2% above and below the point of connection of case fit thereto, on the ctherside. To insure this balanced condition, I provide an indicator such as a lamp titer other indicating device connected across a portion of thelength of the case 48. ihus if any differenceoi potential exists along the latter, thelamp as will'be illuminated. In such case, the position of the tube iiiwould be altered until a balanced condition is obtained. It is for this purpose that a plurality of-holes 43 are provided in the side wall of the'mold.

The leads is from the thermocouplefi are brought out througl'i the lower half of the coil 23 and are connected to a millivolt' meter' 50, thus giving a ready comparison between the temperature at the center of themass it and the'tempe'ratures of the plates 5 3 and 14.

In a preferred practice of the method of my invention, the mold ii! is filled with a charge of material to be processed, the plate i3 and cover il being removed to permit filling oi the mold. This may readily be accomplished-by withdrawing the upper pins i6 and lifting the cover H and plate I3, the plate It being left in position. When the mold has been filled, the charge is compressed and the plate 13 and cover ll replaced, after which the charge is held under pressure by insertion of the pins to. The charge is then ready to be heated dielectrically to cure the heat-activatable binder on the cork'granulesforming the charge.

The tube 32 having been insertedthrough the mold wall into the mass Hi, the switch 32 is closed to energize the heaters 29. The plates i3 and M are thereby heated and their temperature begins to rise slowly, being indicated on the meters 31 and 4|. At this time, the oscillator i9 is connected across the tuning coil 22 by closing the contactor 29. Heating of the mass 15 by the dielectric effect is thus initiated and the temperature existing at the'center of the mass is indicated on the meter 55 The average output of the oscillator i9 is controlled by a power reguletor 5! in such manner that the temperature rise in the mass it follows that or the piates i3 and If: as closely as possible. As a result of this procedure, the plates [3 and Ml do not absorb any heat from the mass i5and the temperature of the top and bottom of the latter rises at the same rate as the interior thereof, thus effecting a uniform curing of the binder throughout the mass. When the temperature at the center of the mass 55 has been raised to the desired value, the contactor 2! is tripped to disconnect the oscillator'from thetuning coil-and theswitch 32 is'opened. On withdrawal 'of'the tube 42, the

mass l5, now fully cured, may be removed from the mold.

' It Will be observed that the processing described is not time-controlled but is governed solely by the heating of the mass to the desired final. temperature which is accurately measured and is substantially uniform throughout the mass. The proper curing of the mix is thus insured regardless of variable factors such as atmospheric ternperature and humidity which cannot be accurately controlled except at considerable expense.

The average output of the oscillator or input to the resonant circuit may be controlled in several ways, of which the following are examples. The voltage on the plate of the oscillator tubes may be varied by means of an auto-transiormer in the input side of the rectifier supplying the plate voltage in order to control the output power of the oscillator. A grid-controlled rectifier also allows the voltage applied to the plate of the osoillator tubes to be varied, thereby controlling the oscillator output power. A further method of control is by keying, e., negatively biasin the grid of the oscillator tubes by a vacuum tube switch so that the plate current of the oscillator is blocked for a portion of each cycle thereby altering the average power delivered. Finally, the oscillator may be coupled to the resonant circuit or load inductively or capacitatively, and the power delivered to the latter may be controlled by varying the coupling.

In order to facilitate the processing as just described, I may employ automatic control devices including a differential temperature controller and a straight temperature controller 53. These devices are of known construction and are available on the open market. The controller operates to close one contact when one of two temperatures being measured is above the other and to close another contact when the first temperature is below the second. The controller 53 closes a contact when the temperature under measurement reaches the value for which the controller is set. Leads 36 and 4? extend from the resistor 38 and the resistor respectively, through the tubular conductor of coil 23, to the controller 52. The lead 41 connected to the resistor 45 is similarly connected to the controller 53. The electrical resistance thermometer 38, 45 and 45 thus serve to operate the controllers 52 and 53.

The controller 52 operates through a reversing panel 5i provided with suitable relays and a timer for controlling a motor 56. The motor 58 operates the regulator iii to control the output of the oscillator is by any one of the methods outlined above, except the keying system which may be controlled directly by the reversing panel In Figure 1B the connections between the several elements, whether electrical or mechanical, are indicated by dotted lines.

The controller 52 functions to cause the regulator 5! to increase the output of the oscillator l9, and the increased output is applied to the mass so long as the temperature at the center of the mass I5 is below that of the plate l3, and to reduce the oscillator output whenever the tem perature at the center of the mass exceeds that of the plate. In the keying system, the controller 52 and reversing panel 5? will eiiect a negative biasing of the grids of the oscillator tubes and this will prevent the system from oscillating so long as the negative biasing exists. The timer incorporated in the panel 51 causes the controller 52 to be effective only during predetermined time intervals, e. g., intervals of a few sec..

.onds with inactive periods of the same length therebetween. This on-and-off action of the controller tends to prevent 0ver-correcti0n and hunting. The timer may be a simple motordriven contact-making device connected in series with the reversing relays and the contacts of the controller 52.

As previously indicated, the controller 53 serves to trip the contactor 2| and cut off the oscillator from the tuning coil when the temperature at the center of the mass 15 has risen to the desired maximum value for complete curing of the binder.

In order to permit the use of temperature controllers operating from thermocouples instead of resistance thermometers such as shown at 38, 45 and 45, I provide a modified form of thermoresponsive element 42' shown in Figure 4, with its end in section to enlarged scale. Thi tube has thermocouples 6t and 8! with their hot junctions welded to its end. A thermocouple (not shown) is also substituted for the resistor 38 on plate !3. The thermocouple leads 62 and 63 are brought out through a flexible case 38 similar to that shown at :33 but having an insulated joint 64 intermediate the ends to prevent the flow of direct current. These leads with the leads from the thermocouple on the electrode plate !3 extend through the tubular conductor forming the coil 28. Since the thermocouples 60 and GI are in electrical contact with the tube 42, I make the inner end of case 48' separate from the end connected to coil 20, to isolate the thermocouple substituted for the resistance element 38 from the thermocouple Gil which thermocouples are differentially connected to the recorder 52. A condenser 65 of from 5 to 10 microfarads capacity serves to conduct any high-frequency current flowing in the circuit across the two arms of the bridge (the mass l5 and coil 20) but this should be small in any event and zero when proper balance is obtained. The leads from the thermocouple 6| substituted for the resistance element 45' are connected to the temperature controller 53 which serves to trip the contactor 2| in the manner previously described when the temperature at the center of the mass has risen to the desired maximum value.

Instead of an oscillator having one side grounded, I may use the well-known push-pull circuit. In that case, the thermo-responsive element 42 would have to be inserted with care so as to be at ground potential. The electrode plates l3 and M in such systems would be at substantial voltages above and below ground. The temperature-control leads from the tube 42 would therefore be taken through a grounded shield to the exterior of shield S. The temperature-control and heating-current supply leads for the electrode plates would be taken into opposite ends of the coil 20 and out of the grounded mid-point thereof. I1 the element 42 should be inserted into the mass l5 at; a plane other than the median transverse plane, the leads would then have to be taken out through the coil 20.

It will be apparent from the foregoing that the invention is characterized by numerous advantages of marked importance in the practical application of dielectric heating to industrial processing. In the first place, by comparing the temperature of the mass of material being processed with that of the electrode plates, I am able to heat the mass at substantially the same rate as the plates. The overall heating rate may thus be the maximum consistent with maintaining uniform temperatur conditions throughoutthe :massduring the heating stage of the curing cycle and the length of the cycle is kept at a minimum. Increased efirciency results from utilizing electric ,heaters operating from an-existing supply source for heating the electrode plates because such heaters have an efficiency much higher than that which can be attained inthe heating of material .by the dielectric efiect. The heat which is supplied to the electrode plates by the resistance heaters, of course, prevents them'from absorbing .from. the mass of material the heat generated therein by the dielectric effect. a v

By :the use of atuning coil composed of tubular :conductorand placing therein the leadsv extending to the heaters for the high-voltage electrodes and the thermocouple and resistance thermometer thereof, it is possible. to obtainthe temperature :of the plate and supply energy to ,electrical heating means therefor, without mak- 111g electrical contact with the high-voltage cir- .cuit itself. The heating of the electrode plates thus renderedpossible permits a uniform curing of the dielectric mass not previously attained, be-

cause of the loss of heat from the mass to the electrode plates.

. It will be obvious that,.instead of maintaining the temperature of the mass of dielectric material substantiallythe same as the temperatureof the electrode plates as the heating proceeds, a differential above or below the plate temperature may be maintained should it be .desired that the top and bottom portions of the mass be heated to temperatures higher or slightly lower than that .at the center.

Although I have illustrated anddescribed but apreierred embodiment and practice of myinvention, it will be recognized thatchanges in the construction and procedure disclosed may be made without departing from the spirit of the invention or the-scope oi the appended. claims.

I claim:

1. In a method of'heating a mass or material dielectrically, the steps including disposing the mess between spaced plates, heating saidplates externally, simultaneousl applying a high-frequency alternating electric field between said plates from a source of high frequency voltage to heat mass internally by the dielectriceffeet, and controlling the average power output ofsaid high-frequency source to heat the mass di'electrically at substantially the same ra ie as the rate of'heating of said plates.

2. In a method. of heating amass of material dielectrically, the steps including confining the mass in a mold between spaced plates,"heating said plates externally, simultaneously applying a high-frequencyalternating electric field between said plates from a source of high-frequency voltage to heat said mass internally by the dielectric effect, and controlling the average power output of said high-frequency source in accordance with the temperature of one of the plates.

3. In a method of heating a mass of material dielectrically, the steps including disposing the mass between spaced plates, heating said plates externally to cause the temperature thereof to rise, simultaneously applying a high-frequency alternating electric field between said plates from a source of high-frequency voltage to heat said mass internally by the dielectric effect, and controlling the average power output of said high frequency source in accordance with the temperature rise of one of the plates.

. 4. In :a method of heating a mass of material dielectrically, the steps'including confining the mass between spaced-plates in a mold composed of material having a higher loss factor than the mass, heating saidplates externally to cause the temperature thereofto rise, simultaneously applying a high-frequency alternating electric field between said plates from a source of high-frequency voltage to heat said mass internally by the dielectric efiect, and controlling the average power output of said high-frequency source in accordance with-the temperature rise of one of the plates.

'5. Ina method of heating a mass of-material dielectrically, the steps including disposing the mass between spaced electrode plates, inserting into said mass-an element responsive to the temperature of said mass, which element is electrically coupledtoa second element responsive to the temperature of oneof said electrode plates, heating said plates externally to cause the temperature thereof I torise, simultaneously applying a high-frequency alternating electric field between said plates from a source of high-frequency voltage to heat said mass internally by the dielectric effect, and controlling the average power output of said-high-frequency source in accordance with differences in temperature of said two thermo-responsive elements to heat the mass dielectrically atsubstantially thesame rate as the rate of heating. of said plates.

6. The method defined by claim 5 characterized by said elementresponsive to the temperature of said mass being inserted. substantially at the median transverse planethrough'the mass parallel to the plates.

7. Apparatus for dielectrically heating a mass of material comprising a pair of electrode plates on opposite sides of the mass, an inductance composed of tubularconductor connectedacross said plates, a source of high-frequency alternating voltage for said inductance, vtherino-resp r 1sive means insertable into said mass, conductors extending from said means entering said tubular conductor atsubstantially the mid-point of said inductance and-extending through the tubular conductor, and atubular case extending laterally from said tubular conductor, the conductors from said thermo-responsive means being enclosed in said case.

8. The apparatus defined in claim "I characterized by means for indicating a difference in voltage between spaced points on said tubular case.

9. Apparatus for dielectrically heating a mass of'material comprising a-mold for confining the mass, a pair of electrode plateson opposite sides of the mass, an inductance composed of tubular conductor connected across said plates, a source of high-frequency alternating voltage for said inductance, thermo-respcnsive means insertable into said mass through an opening provided in said mold, conductors extending from said thermo-responsive means entering said tubular conductor at substantially the mid-point of said inductance and extending through the tubular conductor, and a tubular case extending laterally from said tubular conductor, the conductors from said therrno-responsive means being enclosed in said case.

10. Apparatus for dielectrically heating a mass of material comprising a pair of electrode plates on opposite sides of the mass, an inductance connected across said plates, a source of high-frequency alternating voltage for said inductance,

heating means for said plates to cause the tern perature thereof to rise, a thermo-responsive means disposed adjacent one of said plates, thermo-responslve means adapted to be dispos in said mass, and means actuated by both said thermo-responsive means for so controlling the average power output of said source as to cause the temperature of the mass to conform sub stantially to that of said one of said plates.

11. Apparatus for dielectrically heating a mass of material comprising spaced electrode plates, means for heating said electrode plates to cause the temperature thereof to rise, and means for dielectrically heating a mass of material be" tween said plates in accordance with the temperature attained by one of said plates compris ing an inductance connected across said electrode plates, a source of high-frequency alternating voltage for said inductance, and thermoresponsive means for controlling the average power output of said source to cause the team perature of the mass to conform substantially to that of one of said plates as the temperature of one of said plates rises.

12. In a method of heating a mass of rial dielectrically, the steps including disposinr' the mass between spaced plates, first heating the top and bottom portions of the mass externallyv by heating said plates externally to cause the temperature thereof to rise and contiuiuingheat-- ing from said plates to the top and bottom per-- tions of the mass adjacent said plates, then heat ing the interior of said mass dieleotrically at substantially the same rate of rise as the rate of rise of the plates, and continuing to heat the mass substantially uniformly throughout until it has reached a predetermined temperature.

13. Apparatus for dielectrically heating a mass of material comprising a pair of electrode plates on opposite sides of the mass, an inductance connected across said plates, a source of highfrequency alternating voltage for said induct ance, heating means for said plates to cause the temperature thereof to rise, thermo-responsive means disposed adjacent one of said plates, thermo-responsive means adapted to be disposed in the mass, means actuated by both said thern w responsive means for controlling the avera e power output of said source to cause the temperature of the mass to rise in conformity with the rise in temperature of said plates, and means for interrupting the supply of high-frequency voltage to said plates when the tei'iiperature of said mass has reached a predetermined alue.

14. In a method of heating a mass of material dielectrically, the steps comprising disposin the mass between spaced plates, heating said plates externally, simultaneously applying a highfrequency alternating electric field between plates from a source of high-frequency voltage controlling the average power output of said high-frequency source to heat the mass dielectrically at substantially the same rate as the rate of heating of said plates, and discontinuing the application of external heat to the plates and the application of the field to the mass when the temperature of the mass has reached a predetermined value.

15. Apparatus for dielectrically heating a mass of material comprising a pair of electrode plates on opposite sides of the mass, anv inductance composed of a tubular conductor connected across said plates, a source of high-frequency alternating voltage for said inductance, thermo-respon sive means insertable into said mass, conductors extending from said means entering said tubular conductor at substantially the mid-point of said inductance and extending through the tubular conductor, and a. tubular case for said thermoresponsive means, said case comprising two sections having an insulated joint therebetween to prevent the flow of direct current therethrough and a condenser connected across said sections to conduct any high-frequency alternating current flowing in said sections.

16. Apparatus for dielectrioally heating mass of material comprising a pair of electrode plates for disposition on opposite sides of the mass connected to a source of high-frequency alternating voltage, heating means for said plates to cause the temperature thereof to rise, a thermo-couple disposed adjacent to one of said plates, a second thermocouple disposed in the mass between said electrode plates, and means responsive to difference in. temperature between said thermocouple adjacent to said plate and said thermocouple dlsposed in the mass for controlling the average power output of said source to cause the temperature of the mass to rise substantially in, conformity with the rise in temperature of said plates.

GEORGE E. GARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date l,798.6'78 Keller Mar. 31, 1931 2,304,958 Rouy Dec. 15, 1942 2,363,719 Cooper et al. Nov. 28, 1944 2,386,966 MacMillin Oct. 16, 1945 2,388,824 Brown Nov. 13. 1945 FOREIGN PATENTS Number Country Date 699,082 Germany Nov. 22, 1940 OTHER REFERENCES Modern Plastics, June 1944, page 109,

Certificate of Correction Patent No. 2,508,382 May 23, 1950 GEORGE E. GARD It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 26, for cry read very; column 11, line 60, after the word voltage insert a comma;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 15th day of August, A. D, 1950.

[sun] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent N 0. 2,508,382 May 23, 1950 GEORGE E. GARD It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 26, for cry read very; column 11, line 60, after the word voltage insert a comma;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of August, A. D, 1950.

[sun] THOMAS F. MURPHY,

Assistant flommz'ssz'oner of Patents. 

